When a golf club head strikes a golf ball, a force is seen on the club head at the point of impact. If the point of impact is aligned with the center of gravity (CG) of the golf club head in an area of the club face typically called the sweet spot, then the force has minimal twisting or tumbling effect on the golf club. However, if the point of impact is not aligned with the CG, outside the sweet spot for example, then the force can cause the golf club head to twist around the CG. This twisting of the golf club head causes the golf ball to acquire spin. For example, if a typical right handed golfer hits the ball near the toe of the club this can cause the club to rotate clockwise when viewed from the top down. This in turn causes the golf ball to rotate counter-clockwise which can result in the golf ball curving to the left. This phenomenon is what is commonly referred to as “gear effect.” Recent manufacturing techniques that allow for a higher coefficient of restitution (COR) or the use of inverted cone technology (ICT) increase this gear effect.
Bulge and roll are golf club face properties that are generally used to compensate for this gear effect. The term “bulge” on a golf club typically refers to the rounded properties of the golf club face from the heel to the toe of the club face. If a club face is rounded, then the angle that the golf ball leaves the club face relative to the intended target line will be increased for off-center shots. For example, if a golf ball is hit near the heel of the club face, then the ball will leave in an initial direction to the left of the target line. As suggested above, with an off-center heel shot the ball can curve to the right so ideally the two effects will neutralize one another and produce a flight path that lands the ball close to the intended target line.
The term “roll” on a golf club typically refers to the rounded properties of the golf club face from the crown to the sole of the club face. When the club face hits the ball, the ball acquires some degree of backspin. Typically this spin is greater for shots hit below the center line of the club face than for shots hit above the center line of the club face.
Recent advances in manufacturing techniques and materials properties have enabled golf club manufacturers to increasingly vary the weight, shape and center of gravity of golf club heads. These advances allow the moment of inertia (“MOT”) of the golf club heads to be increased, as disclosed for example in U.S. Pat. No. 6,648,773 B1 to Evans. Thus, the club head twists less when it strikes the ball off-center, as described above. This decreased twisting can lead to decreased ball spin, depending on the location of ball contact. Recent developments in high MOI clubs having conventional face configurations can lead to greater deviation for shots away from center face.